1. Field of the Invention
The present invention relates generally to a precise alignment system and particularly to an alignment system that can provide an automatic detection in a proximity and projection lithography system to transfer a high resolution pattern from a mask to a substrate.
2. Description of the Prior Art
The miniaturization of integrated circuits has created a constant demand to precisely align a mask with a substrate for each of the lithographic steps in the fabrication of an integrated circuit on a substrate. The higher the accuracy of alignment, the greater capability is provided to reduce the minimum feature size of an integrated circuit.
In providing integrated circuits having feature sizes below 1.0 .mu.m, the prior art has adopted various interferomic techniques, such as disclosed in D. C. Flanders et al., "A New Interferometric Alignment Technique," Vol. 31, No. 7, Oct. 1, 1977, pp. 426-428, that discloses the basic interferomic technique. In this regard, diffraction gratings of identical period are provided on the facing surfaces of a mask and substrate. A beam of laser light is normally directed to the diffraction grating planes with the result that diffracted light is returned at discrete angles from the incident laser beam. Generally, those light beams experiencing the smallest unit of an angular diffraction, this is, a net first order diffraction, are utilized in the Flanders et al. technique. Measurements of the relative difference in the plus and minus first order diffraction group intensities are used to obtain an indication of the alignment of the mask and substrate gratings. Any relative displacement of the mask with respect to the substrate within the period of the gratings will show a corresponding variation in the relative intensities of the first order diffraction groups due to the mutual interference between beams within each group. Theoretically, a zero intensity difference between the plus and minus first order diffraction groups will occur only when the mask and the substrate diffraction grating lines are aligned. The Flanders et al. technique proposed that alignment errors as small as 200 angstroms could be detected.
U.S. Pat. No. 4,596,467 is the work of one of the inventors of the present application, assigned to the same assignee. This patent discusses the various problems that are inherent in the Flanders et al. technique and attempts to address these problems, such as the gap or specific spacing between the mask and substrate that can introduce various problems that prevent any discrimination between mask and substrate grating alignment errors from the variables of the spacing between the mask and the substrate.
This patent teaches the use of dissimilar grating periods for the mask and substrate gratings. An indication of alignment between the mask and the substrate gratings is obtained by comparing the relative apparent movement of fringes produced by the first order of diffracted light beams that are effectively diffracted only by the mask with that due to be effectively diffracted by the substrate. The U.S. Pat. No. 4,596,467 embodiments require precise and complicated optics that are cumbersome to align in a vacuum lithography environment.
The articles "A Dual Grating Alignment Technique For X-Ray Lithography" by Kinoshita et al., J. Vac. Sci. Technol., B, Vol. 1, No. 4, Oct.-Dec. 1983, pg. 1276, and "A High-Resolution Laser Scale Interferometer" by Iwaoka et al., SPIE, Aug. 1984, are cited of general interest.
Finally, there have been attempts to provide a precise alignment system for positioning singular objects. As with the proximity and projection lithography systems, there is still a demand to increase the accuracy of such systems.
Thus, there is still a demand in the prior art to provide improved interferomic alignment systems.